The invention relates to a valve for controlling the pressure of a fluid, such as air, which is applied to a brake cylinder of a vehicle, particularly, a railway car, and which can be continuously variable in dependence on the load on such vehicle.
Braking systems for railway cars in which the braking force applied to the wheels of all the trucks of a car or to the wheels of individual trucks dependent upon the load on the car or on the individual trucks are known in the art. See, for example, U.S. Pat. Nos. 1,615,366; 4,648,661; 4,775,197 and the patents cited in the application therefor; and U.S. Pat. Nos. 4,826,259 and 5,039,174.
In general, such prior art valves provide reduced braking force when the railway car is empty or only lightly loaded and require a reservoir to which air is diverted under such conditions. When the car loading is increased and reaches a predetermined value, e.g. a load which causes one-half of the car spring travel, the valves change over to a condition in which full braking force is applied. In other words, such valves provide only two different braking forces, a low force for a light load and a high force for a heavy load. Such valves also require an equalizing reservoir or canister of an internal volume sufficient to reduce the air pressure supplied to the valve to a value at the brake cylinder which will produce the desired low force.
In cases such as coal or other bulk commodity cars which generally are fully loaded or completely empty when moved, current systems are satisfactory. In cases of container, box, gondola or bulk type cars which carry light materials when fully loaded in one service, and dense or heavy material when loaded with some other commodity in another service, the current, strictly empty or load devices, will not provide the optimum braking performance.